Gas sensor having a laminate comprising solid electrolyte layers and alumina substrate

ABSTRACT

A gas sensor ( 1   a ) having a laminate including an alumina substrate ( 11 ) having a heating resister ( 115 ) embedded in the alumina substrate ( 11 ); a first oxygen-ion conductive solid electrolyte layer ( 131 ) containing zirconia and alumina and partly constituting an oxygen-detecting cell ( 13 ) and the first solid electrolyte layer ( 131 ) being laminated with said alumina substrate ( 11 ); a second oxygen-ion conductive solid electrolyte layer ( 121 ) containing zirconia and alumina and partly constituting an oxygen-pumping cell ( 12 ); an ion-leakage preventing ceramic spacer ( 143 ) for preventing oxygen-ions from leaking from the second oxygen-ion conductive solid electrolyte layer ( 121 ) to the first oxygen-ion conductive solid electrolyte layer ( 131 ), the spacer ( 143 ) being laminated between the first and second oxygen-ion conductive solid electrolyte layers ( 131, 121 ); and a gas-diffusion space ( 141 ) formed between an electrode ( 133 ) of the oxygen-detecting cell ( 13 ) and an electrodes ( 126 ) of the oxygen-pumping cell ( 12 ). Furthermore, the laminate ( 1   a ) is co-fired. Preferably, the zirconia contained at least in the second solid electrolyte layer is made of partially stabilized zirconia, the phase formed in the zirconia consisting essentially of tetragonal and cubic phases. Additionally, an ionic migration-preventing electrode ( 117 ) is optionally embedded in the alumina substrate ( 11 ) for preventing metal ion migration.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a gas sensor having alaminate or multilayer structure comprising at least two solidelectrolyte layers and an alumina substrate, for use in an internalcombustion engine, particularly to a full-range air-fuel ratio sensor(or rather a universal exhaust gas oxygen sensor) capable of measuringair-fuel ratio of an internal combustion over the entire range thereof,a nitrogen oxide gas sensor, a flammable gas sensor capable of measuringcarbon monoxide or hydrocarbon, a compound gas sensor capable ofmeasuring plural gases selected from oxygen, nitrogen oxides, carbonmonoxide, hydrocarbon gas and other gases.

Specifically, the present invention relates to a gas sensor having aco-fired laminate of at least two zirconia solid electrolyte layers forelectrochemical cells and an alumina substrate for embedding a heatingresistor, for use, for instance, as a full-range air-fuel ratio sensorfor controlling an air-fuel ratio from fuel-lean to rich in an internalcombustion engine control, etc.

2. Description of the Related Art

Increasingly strict regulations have been imposed on the emissionquantity of harmful matter (e.g., hydrocarbon gas, carbon monoxide, andnitrogen oxides) contained in exhaust gas discharged from an internalcombustion engine of an automobile, etc. Moreover, in view of thegreenhouse effect and other problems, the necessity to reduce theemission of carbon dioxide has arisen, thereby raising an urgent needfor a method of further reducing consumption of fuel by internalcombustion engines.

Under such circumstances, more strict requirements have been imposed ongas sensors, which are indispensable for decreasing harmful mattercontained in exhaust gas and improving fuel efficiency of the internalcombustion engines. In particular, in recent years, demand has arisenfor a gas sensor that can activate quickly and can save electric power,while providing improved performance and reliability as well asreduction in size and cost.

U.S. Pat. No. 4,765,880 discloses a configuration of a two-cell gassensor, including an oxygen-pumping cell and an oxygen-detecting cell.This type of two-cell gas sensor enables full-range measurement ofair-fuel ratio of an internal combustion engine of an automobile, tothereby improve fuel efficiency of the internal combustion engine.

USPAP 2001/0047937 A1 discloses a multilayered air-fuel ratio sensorincluding solid electrolyte substrate layers and at least oneheterogeneous boundary layer for absorbing thermal shock or stressinterposed between the solid electrolyte substrate layers.

EP 1026502A2 discloses a one-cell type gas sensor including an aluminasubstrate laminated integrally with an oxygen-ion conductive solidelectrolyte layer containing alumina. U.S. Pat. No. 4,733,056 disclosesa technique for preventing ion-migration in a ceramic heater per se.

3. Problems Solved by the Invention

In the case of a plural cell-type sensor such as a full range air-fuelratio sensor that requires an oxygen-pumping cell, anoxygen-concentration detecting cell, a heater for heating the cells anda cavity or space into which oxygen is pumped in or out by the pumpingcell, various problems relating to activation of the sensor cells by theheater under a limited electric power consumption, an oxygen-pumpingcapability of the oxygen-pumping cell, measurement accuracy of theoxygen-detecting cell, reliability of the sensor, etc., arises. This isbecause the structure and function of the plural cell-type sensor areextremely complicated, compared to a single cell-type sensor.

In addition, electrochemical and structural weakness caused by metal-ionmigration, oxygen-ion leakage between the cells, reduction or ratherdeoxidization of a solid electrolyte layer that constitutes theoxygen-pumping cell, etc., will be problematic in this plural-cell typesensor.

Further, when the gas sensor is designed to adopt a laminate (ormultilayer structure) comprising plural zirconia oxygen-ion conductiveceramic layers and an alumina ceramic substrate (for a heating resistorto be embedded therein), etc., a serious problem such as cracks inducedin the laminated zirconia ceramic layers arises due to thermal expansiondifference between the zirconia ceramic layers and the aluminasubstrate.

A conventional two-cell type sensor used in an actual automobilecombustion engine control system has been composed of two portions(namely, zirconia-sensor cells and a heater-embedded alumina substrate)bonded by a comparatively thick glass of about 200 micrometers whichabsorbs stress caused by the thermal expansion difference therebetween.This means at least two firing processes (one for the zirconia cells andthe other for the heater-embedded alumina substrate) are necessary,resulting in a costly sensor with a slow activation of the sensor cellsdue to use of glass having lower thermal conductivity, as compared toalumina. Another conventional two-cell type sensor has been composed ofzirconia-sensor cells and a heater-embedded zirconia substrate, whichalso has a disadvantage in the activation of the sensor cells due to useof the heater-embedded zirconia substrate that has insufficiently lowthermal conductivity, as compared to the alumina substrate.

From a view point of activation of the sensor cells, the heater has beenconventionally attached closer to the oxygen-pumping cell than to theoxygen-detecting cell. In this manner, the temperature of theoxygen-pumping cell can be elevated faster than that of theoxygen-detecting cell. This is because the cavity of the two-cell typesensor delays activation of the sensor, as compared to a single celltype sensor. If electric power for heating the heater is increased forquick activation of the sensor cells, durability and endurance of theheater is sacrificed. If the size of the sensor is made too small, thepumping capability of the oxygen-pumping cell becomes insufficient foraccurate determination of the air-fuel ratio of the internal combustionengine. Because of these disadvantages, prior investigators have notbeen widely successful in providing or incorporating a two-cell type gassensor into an automobile internal combustion engine and/or an exhaustgas control system thereof.

SUMMARY OF THE INVENTION

The present invention can solve the above-described problems andpotential problems of the prior art, and an object of the invention isto provide a two-cell type gas sensor of having a laminate, which sensoris advantageous in terms of size, structural strength, sensoractivation, electric power consumption, reliability, measurementaccuracy, electrochemical endurance, durability and/or manufacturingcost.

Another object of the invention is to provide a ceramic gas sensorcomprising a laminate of at least two oxygen-ion conductive ceramiclayers and an alumina substrate embedding a heating resistor therein,which sensor performs quick activation, stable and accurate measurement,and structural endurance against electrochemical degradation under asevere thermal cycling condition.

Yet another object of the invention is to provide a gas sensor having aco-fired laminate of at least two oxygen-ion conductive ceramic layersfor sensor cells and an alumina substrate embedding a heating resistor,which sensor has high structural strength and electrical reliability anddoes not malfunction due to ionic migration in the alumina substrate anddeoxidization of electrodes of the cells.

The above objects of the present invention have been achieved byproviding two kinds of gas sensors comprising a laminate of at least twosolid electrolyte layers and an allumina substrate; one kind with anionic migration-preventing electrode and the other without such an ionicmigration-preventing electrode.

A gas sensor comprising a laminate, according to the present invention,has at least two of the following features (A) to (T) followed by atleast one advantage or advantageous reason as described below. Referencenumerals inserted herein or hereafter are only for the purpose ofexplaining the invention and do not limit the invention to specificdrawings.

(A): An alumina substrate 11 is laminated onto an oxygen-detecting cell13 and has a heating resister 115 embedded in the alumina substrate 11.Notably, as described in detail below, when an ionicmigration-preventing electrode 117 is incorporated or embedded in thealumina substrate 11, the content of alumina in the alumina substrate 11may be varied, for instance, from about 70 to 100% by weight. When theionic migration-preventing electrode 117 is not present, the content ofalumina in the alumina substrate 11 should be more than 99% by weight ofalumina, preferably more than 99.9% by weight, or most preferably morethan 99.99%.

An advantage of feature (A) is that good thermal transfer from theheating resistor 115 to an oxygen-pumping cell 12 through an insulatinglayer 111 of the alumina substrate 11 and through an oxygen-detectingcell 13 is attained and quick activation of the sensor cells 12, 13 isaccomplished. This is because alumina has a higher thermal conductivitythan other insulating oxide material and the alumina substrate 11 isco-fired with first and second oxygen-ion conductive solid electrolytelayers 131, 121 as described below.

(B): The first and second oxygen-ion conductive solid electrolyte layers131, 121, partly constituting an oxygen-detecting cell 13 and anoxygen-pumping cell 12, respectively, contain zirconia and alumina so asto be laminated and co-fired with the alumina substrate 11.

An advantage of feature (B) is that alumna grains, when they arecontained with zirconia in the solid electrolyte layers 131, 132, worknot only as a grain-growth inhibitor for zirconia grains that arepartially stabilized by a stabilizer such as yttria during sintering,but also as a phase-transformation suppressor for zirconia phase aftersintering. When the average grain size of the alumina and that of thepartially stabilized zirconia contained in the second oxygen-ionconductive solid electrolyte layer 121 of the oxygen-pumping cell 12 arecontrolled, as explained below, to be less than 1 micrometer and 2.5micrometers respectively after sintering, the phase transformation ofzirconia phase causing structural weakness of the laminate (1 a) iseffectively suppressed. Notably, the thermal expansion coefficient ofalumina is about 7.7–8.1×10⁻⁶/K and that of partially or whollystabilized zirconia consisting substantially of tetragonal and/or cubicphase is about 9–12.6×10⁻⁶/K, in a temperature range of 298–1150° K.

Another advantage of feature (B) is that quick activation of the pumpingcell 12 is improved by including alumina in the cell electrolyte layers121, 131. This is because the thermal conductivity of alumina isoutstandingly high as compared to zirconia. The thermal conductivity ofalumina is more than 10 times higher than that of zirconia at 100degrees centigrade (Celsius).

(C): An ion-leakage preventing ceramic spacer 143 for preventingoxygen-ions from leaking from the second oxygen-ion conductive solidelectrolyte layer 121 to the first oxygen-ion conductive solidelectrolyte layer 131 is interposingly co-fired between the first andsecond oxygen-ion conductive solid electrolyte layers 131, 121.

An advantage of feature (C) is that accurate measurement is attained bypreventing oxygen-ions from leaking across the solid electrolyte layers131, 121. Without such an ion-leakage preventing ceramic spacer 143,when the oxygen-pumping cell 12 pumps oxygen from or into a diffusionspace 141, the second oxygen-ion conductive layer 12 leaks itsoxygen-ions into the first oxygen-ion conductive layer 131, causingerroneous measurement of electromotive force produced across anoxygen-detecting electrodes 133 and a reference electrode 136 of theoxygen-detecting cell 13.

A preferable material for the ion-leakage preventing ceramic spacer 143is alumina or alumina containing less than 20% by weight of zirconia.This is partly because the alumina grains are included in the first andsecond solid electrolyte layers 131, 121 so as to match a thermalexpansion thereof to the alumina substrate 11, partly because thealumina per se does not greatly harm internal resistance of theoxygen-pumping cell 12 comprising zirconia while other material such assilica does, and further because a voltage is applied across electrodes123, 126 of the oxygen-pumping cell 12 in order to independently measurean oxygen-pumping ionic current flowing across the electrodes 123, 126.Notably, this oxygen-pumping ionic current is used as an indicator ofcombustion state (fuel-rich to lean) of the exhaust gas.

(D): A gas-diffusion space 141 is formed between an electrode 133 of theoxygen-detecting cell 13 and an electrode 126 of the oxygen-pumping cell12, advantageously featuring a distance therebetween of 20–80micrometers. This gas-diffusion space is necessary for a full rangeair-fuel ratio sensor, a NOx sensor for detecting nitrogen oxide, etc.In a fuel-rich condition (i.e., an oxygen-scarce state), the amount ofoxygen pumped into the gas diffusion space 141 until when apredetermined oxygen partial pressure is detected by theoxygen-detecting cell 13 is measured. In a fuel-lean condition (i.e., anoxygen-abundant state), the amount of oxygen pumped out of the oxygenspace 141 until when the predetermined oxygen partial pressure isdetected by the oxygen-detecting cell 13 is measured. In this way, theoxygen amount pumped into or pumped out of the gas diffusion space 141indicates the combustion state (rich to lean) of an internal combustionengine. If the distance is more than 80 micrometers, the temperaturedifference between the oxygen-detecting cell 13 and the oxygen-pumpingcell 12 becomes too large. This results in erroneous measurement of anamount of the gas of interest, and in addition, the oxygen-pumping cell121 looses its pumping capability. If the distance is less than 20micrometers, the measurement amount of, for instance the exhaust gas istoo limited or insufficient resulting in erroneous or inaccuratemeasurement of the gas amount.

(E): A laminate of the alumina substrate 11, the first and secondoxygen-ion conductive solid electrolyte layers 131, 121, and theion-leakage prevention spacer 143 is co-fired or rather simultaneouslysintered, such that the second oxygen-ion conductive solid electrolytelayer 121 that partly constitutes the oxygen-pumping cell 12 andcontains zirconia and alumina is laminated on the ion-leakage preventingceramic spacer 143, the alumina substrate 11 embedding a heatingresistor for heating or activating the second oxygen-ion conductivesolid electrolyte layer 121 is laminated on the first oxygen-ionconductive solid electrolyte layer 131 that partly constitutes theoxygen-detecting cell 13, and the ion-leakage preventing ceramic spacer143 for preventing oxygen-ions from leaking from the second oxygen-ionconductive solid electrolyte layer 121 to the first oxygen-ionconductive solid electrolyte layer 131 is laminated between said firstand second oxygen-ion conductive solid electrolyte layers 131, 121.

An advantage of the co-fired laminate (1 a) is dimensional compactnessor size reduction of the gas sensor. The co-fired laminate improvesthermal conduction of heat from the heating resistor 115 to theoxygen-pumping cell 12, heating efficiency of the heating resistor 115for activation of the oxygen-detecting cell 13 and oxygen-pumping cell12 and electric power consumption by the heating resistor 115, comparedto a non-co-fired laminate.

Co-firing as used herein means simultaneously firing or sintering agreen (unfired) laminate comprising a green alumina substrate and greenoxygen-ion conductive solid electrolyte layers under a common firingcondition.

(F): An ionic migration-preventing electrode 117 for preventingdeterioration and/or electrical disconnection of the heating resistor115 is advantageously embedded or incorporated in the alumina substrate11, wherein the electric potential of said ionic migration-preventingelectrode 117 is equal to or lower than the lowest electric potential ofany part of the heating resistor 115.

So long as the electric potential of the ionic migration preventingelectrode 117 is maintained equal to or lower than that of any portionof the heating resistor 115, no metal ions migrate toward the heatingresistor 115 under a voltage applied across the heating resistor 115 athigh temperature. The ionic migration-prevention electrode 117 draws orgathers the migrating metal ions, and vicariously protects the heatingresistor 115 from electrochemical deterioration or electricaldisconnection by the migrated metal ions. Notably, this ionic migrationoccurs especially at a high exhaust gas temperature of more than 700° C.At such high temperature, the metal ions including alkaline and/oralkaline earth metal ions such as Mg and Ca constituting inorganicbinders such as MgO and CaO contained in the alumina substrate 11 or ina bonding material for the alumina substrate 11 and the oxygen ion-ionconductive solid electrolyte layer 131 migrate through the aluminasubstrates and gather around the lowest electric potential portion.

(G): Specifically, the electric potential to be applied at the ionicmigration-preventing electrode 117 is maintained to be equal to or lowerthan an electric potential at a position connecting the heating resistor115 and leads 116 of the heating resistor 115. Since the leads 116 aredesigned to be wider or thicker in width or thickness than the heatingresistor 115 and the temperature of the leads 116 is lower than theheating resistor 115, the electrical disconnection of the leads 116would not occur even if some metal ions migrate or gather around one ofthe leads 116.

(H): The ionic migration-preventing electrode 117 is importantlypositioned between the heating resistor 115 of the oxygen concentrationcell 13 and an outer surface of the alumina substrate 11. In otherwords, the ionic migration-prevention electrode 117 incorporated orembedded in the alumina is most preferably placed between the heatingresistor 115 and the oxygen-ion conductive solid electrolyte layer 131,according to an aspect of the invention.

Since the migrated ions gather around the ionic migration preventingelectrodes 117 and combine with oxygen newly forming a glassy phasearound the ionic migration-preventing electrode 117, the alumina ceramic11 surrounding the ionic migration preventing electrode 117 is weakenedor degraded in strength. Since such a degrading glassy phase which islow in insulating ability is easily formed inside the alumina substrate11 that normally contains a considerable amount of the inorganic binder,cracks tend to occur where the glassy phase is formed inside the aluminasubstrate 11, in addition to insulation failure between the heatingresistor 115 and the reference electrode 136 of the oxygen-detectingcell 13. In a worst case when an ionic migration-preventing electrode117 is positioned between the heating resistor 115 and theoxygen-detecting cell 13, the heating resistor 115 might be separatedfrom the oxygen-detecting cell 13, or a serious problem such as a sensormalfunction and loss of control in an actual exhaust gas control systemwould arise due to insulation lost between the heating resistor 115 andthe oxygen detecting cell 13.

In the case, according to an aspect of the invention, when the ionicmigration-preventing electrode 117 is disposed between the heatingresistor 115 and the outer surface of the alumina substrate 11 such thatthe degrading glassy phase to be formed around the ionicmigration-preventing electrode 117 is formed near or at the outersurface of the alumina substrate 11 on which surface theoxygen-detecting cell 13 is not laminated, even if cracks occur due tothe glassy phase newly formed by the migrated metal ions, the heatingresistor 115 that performs an important role for activating the sensorcells 13, 12 does not separate from the oxygen-detecting cell 131 andthe sensor will not malfunction nor lose accuracy in measuring the gasamount.

Another advantage of the feature (H) is that quick activation of thesensor or rather quick transfer of thermal energy from the heatingresistor through the oxygen-concentration cell 13 to the oxygen pumpingcell 12 is attained, since the ionic migration preventing electrode 117is not positioned between the heating resistor 115 and thereby theglassy phase that slows thermal transfer is not newly formedtherebetween.

This feature (H) characterized in that an ionic migration-preventingelectrode 117 is positioned between a heating resistor 115 and an outersurface of an substrate 11, can be applied to any high temperature gassensor having a laminate of an oxygen-ion conductive solid electrolyteconstituting an oxygen-detecting cell and an alumina substrate embeddinga heating resistor.

(I): The second oxygen-ion conductive solid electrolyte layer 121contains alumina in an amount less than that of the first oxygen-ionconductive electrolyte layer 131. In other words, the content of thealumina contained in the first oxygen-ion conductive solid electrolytelayer 131 is higher than that of the alumina contained in the secondoxygen ion conductive solid electrolyte layer 121.

Advantage of this feature (I) is focused on prevention of possiblefracture of the oxygen-pumping cell 12 during the time at which theheating resistor 115 is forced to quickly elevate the temperature of theoxygen-pumping cell 12 through the oxygen-detecting cell 13. Thisfracture may be accompanied by “blackening” or reduction (deoxidization)of the second oxygen-ion conductive solid electrolyte layer 121 when thetemperature difference between the two laminated cells 13, 12 is toolarge or when the temperature-elevating transient period such as afteractivation of the cells 13, 12 is started at a very cold temperature.When the sensor laminate (1 a) comprising the two cells 13, 12 is verycold and needs to be heated quickly for activation by the heatingresistor 115, the oxygen-detecting cell 13 that is closer to the heatingresistor 115 reaches its activation temperature faster than theoxygen-pumping cell 12 does. The oxygen-detecting cell 13 electricallyrequests or orders the oxygen-pumping cell 12 to pump in or pump oxygenout of the gas diffusion space 141 through a control circuit, even ifthe oxygen-pumping cell 12 is not fully activated and not ready to pumpoxygen. This is when the oxygen-pumping cell 12 deprives oxygen ionsfrom the zirconia of its oxygen-ion conductive solid electrolyte layer,instead of pumping oxygen of the diffusion space 141, and causes“blackening” or reduction of the oxygen-ion conductive solid electrolytelayer 121 constituting the pumping cell 12 and possibly fractures theoxygen-pumping cell 12.

A higher content of alumina in the first oxygen-ion conductive solidelectrolyte layer 131 advantageously increases the internal resistanceof the oxygen-detecting cell 13 so that it is higher than that of theoxygen-pumping cell 12, and slows down or paces down the activation ofthe oxygen-detecting cell 13 so as to match it with that of theoxygen-pumping cell 12. This matching of activation between the twocells 13, 12 incorporated in the sensor laminate (1 a) is advantageouslyattained and stabilized by inclusion of more alumina into the firstoxygen-ion conductive electrolyte layer 131 than into the secondoxygen-ion conductive solid electrolyte layer 121, since a feedbackcontrol circuit is used across the cells 13, 12, based on a temperaturemeasured based on the internal resistance of the oxygen-detecting cell.

(J): Specifically, the amount of alumina contained in the oxygen-ionconductive solid electrolyte layer 121 that constitutes the oxygenpumping cell 12 is less by at least 5% by weight, or preferably less byat least 10% by weight, than that of the alumina contained in theoxygen-ion conductive solid electrolyte layer 131 constituting theoxygen-detecting cell 13.

An advantage of this feature (J) is similar to those described for theabove feature (I), but the advantage becomes more remarkable when thefirst oxygen-ion conductive solid electrolyte layer 131 contains 10 to80% by weight of alumina and 20 to 90% by weight of zirconia.

Further advantage of this feature (I) is that measurement of theelectromotive force produced across electrodes 133, 136 of the firstoxygen-detecting cell 13 becomes more stable than measurement of thesame across an oxygen-detection cell that uses an oxygen ion conductiveelectrolyte layer containing substantially no alumna or rathercontaining less than 5% by weight of alumina. This is because theincrease of the internal resistance of the first oxygen-detecting cell13 thus realized stabilizes measurement of the electromotive forceproduced by the oxygen detecting cell 13, the electromotive forceactually being measured as a voltage detected across a outer resistorattached across the oxygen detecting cell 13.

Preferably, the second oxygen-ion conductive solid electrolyte layer 121that constitutes the oxygen-pumping cell 12 contains 60–90% by weight ofzirconia and 10–40% by weight of alumina, while the first oxygen-ionconductive solid electrolyte layer 131 that constitutes theoxygen-detecting cell 12 contains preferably 40–80% by weight ofzirconia and 20–60% by weight of alumina.

Specifically, the second oxygen-ion conductive solid electrolyte layer121 that constitutes pumping cell 12 contains alumina in an amount lessby 10–50% by weight than the first oxygen-ion conductive solidelectrolyte layer that constitutes the oxygen-detecting cell 131.

(K): The first and second oxygen-ion conductive solid electrolyte layers131, 121 contain 10–80% by weight of alumina, respectively, and theaverage grain size of alumina contained in the first and secondoxygen-ion conductive solid electrolyte layers 131,121 is less than 1micrometer.

An advantage of this feature (K) is effective prevention of a phasetransformation of zirconia contained in the laminated oxygen-ionconductive solid electrolyte layers 131, 121 in an actual thermalcycling environment, and prevention of cracks induced in the sensorlaminate (1 a) due to thermal expansion difference between the laminatedsolid electrolyte layers 131, 121 and the alumina substrate 11. Thisfeature (K) is important to the second oxygen-ion conductive solidelectrolyte layer 121 that constitutes the oxygen-pumping cell 12. Fineralumina grains better prevent the phase transformation of the partiallystabilized zirconia in the second oxygen-ion conductive solidelectrolyte layer 121.

(L): The zirconia included in the second oxygen-ion conductive solidelectrolyte layer 121 is a partially or wholly stabilized zirconia.Preferably, zirconia that substantially consists of a partiallystabilized zirconia with a phase (or phases) of the zirconiasubstantially consisting of tetragonal and cubic phases, containing nomonoclinic phase or rather less than 5% by weight of a monoclinic phase,is used in the first and second oxygen-ion conductive solid electrolytelayers 131, 132.

An advantage of this feature (L) is strength and endurance (or ratherdurability) of the two cells 13, 12 in a practical thermal cyclingenvironment. If a monoclinic phase of zirconia is containedsubstantially (more than 5% by weight) in the oxygen-ion conductivelayers 131, 121, a zirconia phase transformation reversibly occurs frommonoclinic to tetragonal under thermal cycling and thereby micro-cracksor structural weakness of the first and second oxygen-ion conductivesolid electrolyte layers 131, 121 that are co-fired and multi-layeredwith the alumina substrate 11 appears. Although the thermal expansioncoefficient of the monoclinic phase is lowest among the cubic,tetragonal and monoclinic phases and is lower than that of alumina, useof the monoclinic phase as an adjuster for thermal expansion differencebetween the alumina substrate and the solid electrolyte layers 121, 131,especially use thereof in the second-oxygen ion conductive solidelectrolyte layer 121 of the oxygen-pumping cell 12, is avoided becausethe monoclinic to tetragonal phase transformation is detrimental todurability of the sensor laminate (1 a). Without the alumina grains, thereversible monoclinic to tetragonal phase transformation occurs at atemperature of more than 900 degrees centigrade, and the tetragonal tomonoclinic phase transformation occurs under the thermal cyclingenvironment at more than 200 degrees centigrade.

A preferable phase ratio of cubic phase to tetragonal phase formed inzirconia of the solid electrolyte layers 131, 121 is from 1:4 to 2:1,more preferably from 1:3 to 3:1, or most preferably 3:7 to 1:1. Theoptimum phase ratio varies, depending on an amount of the aluminaincluded in the zirconia electrolyte layers 131, 121. In this phaseratio range formed with the alumina grains and partially stabilizedzirconia, a phase transformation of zirconia from tetragonal tomonoclinic, otherwise drastically occurring at a temperature of morethan 200 degrees centigrade (Celsius) especially under a humidenvironment, is effectively suppressed, according to an aspect of theinvention. The phase transformation (also called a phase transition)between tetragonal and monoclinic phases accompanies a volume change ofzirconia per se, and is detrimental to the strength and durability ofthe sensor laminate (1 a). The above phase ratio can be determined by aknown method, e.g., by analyzing the values of X-ray diffraction peakintensity with respect to monoclinic, tetragonal phase and/or cubicphase.

(M): A reference electrode 136 that constitutes the oxygen-detectingcell 12 and directly faces the alumina substrate 11 is a porouselectrode capable of storing oxygen therein. The stored oxygen can beused as referential oxygen with its partial pressure controlled to beconstant by flowing a very small current across the electrodes 133, 136.Excess stored oxygen in the porous electrode is ventilated outside thesensor laminate (1), through a porous lead 127 thereof that is led tooutside the laminate (1 a). An advantage of this feature is thatsubstantially only oxygen can be designed to pass through lead 137 ofthe reference electrode that works as channel (16) for ventilatingoxygen outside the sensor laminate (1 a). This lead 137 per se works asa channel (16) for draining or ventilating oxygen. Contaminants such aswater do not reach the reference electrode to affect the function of thereference electrode 136 because the oxygen partial pressure of theoxygen in the reference electrode can be elevated by the small currentflowing across the electrodes 133, 136. With this feature isincorporated in the laminate, accurate measurement is improvinglyattained.

(N): Reduction-preventing insulative layers 128,138 are importantlyprovided between the leads 127,124 of the oxygen-pumping cell 12 and theoxygen-ion conductive solid electrolyte layer 121.

An advantage of feature (N) is prevention of the second oxygen-ionconductive layer 121 from reduction (or rather deoxidization) ofzirconia contained in the second oxygen-ion conductive solid electrolytelayer 121 surrounding the leads 127, 124. Without such areduction-prevention insulative layer, deoxidization of zirconia occurssince a voltage is applied across the leads 124, 127 in order to pumpoxygen in or out of the diffusion space 141 by the oxygen-pumping cell12. Notably, “blackening” (i.e., deoxidization) occurs around theelectrode 127 in a fuel-lean state and occurs around the electrode 124in a fuel-lean state, depending on the applied voltage polarity. Theinsulative layer 128 preferably comprises alumina.

Another reduction-preventing insulative layer 138 may be providedbetween the lead 134 of the oxygen-detecting cell 13 and the solidelectrolyte layer 131. Although the lead 134 thereof is not so seriouslyreduced or deoxidized as compared to the leads 124, 127 of theoxygen-pumping cell 12, accurate measurement of electromotive forcedetected across the detecting electrode 133 and the reference electrodeof the oxygen-detecting cell 13 is improved.

(O): The alumina substrate 11 in which the heating resistor 115 isembedded contains at least 99% or preferably 99.9% by weight of alumina.

Feature (O) becomes very advantageous in preventing deterioration orelectrical disconnection of the heating resistor 115 and in decreasingformation of the glassy phase weakening the alumina substrate 11. Anadvantage of feature (O) is that the need for a costly platinum ionicmigration-preventing electrode 117 can be eliminated. Another advantageis size compactness of the laminate (1 a) and thermal transferefficiency of heat generated by the heater 115 thorough the aluminasubstrate 11.

(P): A reinforcing insulative cover 152 that reinforces the secondoxygen ion-conductive layer 121 and protects the lead 124 of theoxygen-pumping cell 12 is used. This feature (P) is advantageous interms of endurance or durability of the lead 124 and structural strengthof the sensor laminate (1 a).

(Q): The thickness of the first oxygen-ion conductive solid electrolytelayer 131 constituting the oxygen-detecting cell 13 is advantageously10–200 micrometers, and the thickness of lead 137 of the electrode 136located between the oxygen-ion conductive solid electrolyte layer 131and the alumina substrate 11 is advantageously 1–20 micrometers orpreferably 8–18 micrometers. This feature (Q) leads to an advantage ofsize compactness and preventing cracks of the electrolyte layer 131 aswell as assurance of reliable oxygen-detecting function of theoxygen-detecting cell 13.

(R): The thickness of the second oxygen-ion conductive solid electrolytelayer 121 constituting the oxygen-pumping cell 12 is importantly 30–400micrometers. Since the oxygen-pumping cell 12 is formed outside theoxygen-detecting cell 12 and forming the diffusion space 141 inside, theminimum thickness of 30 micrometer is necessary for structural strength.However, if the thickness exceeds 400 micrometers, de-lamination of thesolid electrolyte layer 121 from the alumina substrate 11 is induced dueto inconsistency of thermal dissipation. This feature (R) leads toreliability of the two-cell type gas sensor and prevents cracking of theelectrolyte layer 121 as well as lends endurance of the oxygen-pumpingfunction of the oxygen-pumping cell 13.

(S): A gas-diffusion passage 142 having a predetermined resistancecontrolling an amount of the molecules of gaseous components enteringthe gas diffusion space 141 is advantageously formed between the gasdiffusion space 141 and the measurement gas outside the sensor laminate(1 a). Specifically, when this gas diffusion passage 142 is formedbetween the first and second oxygen-ion conductive solid electrolytelayers 131 and 132, it is easy to adjust the predetermined resistanceand to make a high quality sensor laminate (1 a).

(T): The area of electrode 133 of the oxygen-detecting cell 13 is 15 to80% of that of the electrode 126 of the oxygen-pumping cell 126. Thisfeature (T) is advantageous in optimizing the internal resistance of theoxygen-detecting cell 13 and in effectively detecting the amount orconcentration of oxygen inside the diffusion space 141 as a function ofthe temperature of oxygen-detecting cell 13.

Among these features (A)–(T), the most important features are (A), (B),(F), (H), (I), (J), (K), (L), (O) and/or a combination thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic transverse sectional view of a gas sensorcomprising a laminate of two sensor cells and an alumina substrate,according to the present invention.

FIG. 2 is a schematic longitudinal sectional view of the gas sensorshown in FIG. 1, as sectioned along a longitudinal center of the gassenor.

FIG. 3 is a schematic perspective view of the gas sensor shown in FIGS.1 and 2, showing its constituent components.

FIG. 4 is a schematic transverse sectional view of another gas sensorembodiment according to an aspect of the present invention.

FIG. 5 is a schematic transverse sectional view of another gas sensorembodiment according to the present invention.

FIG. 6 is a schematic transverse sectional view of another gas sensorembodiment according to the present invention.

FIG. 7 is a schematic transverse sectional view of another gas sensorembodiment according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The gas sensor embodied according to the invention will be described indetail by reference to the drawings. However, the present inventionshould not be construed as being limited thereto. Referring to FIGS. 1and 2, the gas sensor has a sensor laminate (1 a) comprising at leastthree major laminated components; i.e., an alumina substrate 11embedding a heating resistor 115 and two oxygen-ion conductiveelectrolyte layers 131, 121 respectively constituting anoxygen-detecting cell 13 and an oxygen-pumping cell 12.

In an aspect of the invention, the sensor laminate (1 a) has a structuresuch that the first oxygen-ion conducting solid electrolyte layer 131constituting the oxygen-detecting cell 13 is co-fired between thealumina substrate 11 and the second oxygen-ion conductive solidelectrolyte layer 121 constituting the oxygen-pumping cell. In otherwords, the heating resistor 115 firstly activates the oxygen-detectingcell 13 and then the oxygen-detecting cell 13 activates theoxygen-pumping cell 12. Since a comparatively large ionic current flowsthrough the oxygen-pumping cell 12 and therefore the oxygen-pumping cellis more vulnerable than the oxygen-detecting cell 13 in a practicalthermal cycling environment, the oxygen-detecting cell worksadvantageously as a thermal buffer for buffering a sharp temperatureincrease of the oxygen-pumping cell 12 by the heating resistor 115.

One of the important components laminated into the laminate (1 a) byco-firing is an alumina substrate 11 in which a heating resistor 115 forheating and activating the oxygen-detecting cell 13 and the oxygenpumping cell 12 is embedded and optionally an ionic migration-preventionelectrode 117 for preventing the heating resistor 115 from electricaldegradation and/or electrical disconnection is embedded. The otherimportant components laminated in the laminate (1 a) by co-firing are anoxygen-pumping cell 13 and an oxygen-detecting cell 12, in which cells12, 13 alumina grains are contained. Since the alumina grains arecontained in the oxygen-pumping cell electrolyte 121 having a phasesubstantially consisting of cubic and tetragonal phases, a phasetransformation from tetragonal to monoclinic is greatly prevented.

The alumina substrate 11 may be comprised of first, second and thirdco-fired alumina layers 111, 112, 113. As understood from FIG. 3, theheating resistor 115 and two leads 116 thereof, which are made mainly ofplatinum, are disposed between the first and second alumina layers 111,112, and co-fired therewith. The leads 116 of the heating resistor 115are electrically connected to the outer terminal 156(−) 157(+) formed onan outer surface of the third alumina layer 113 via two through-holespenetrating the alumina layers 112, 113.

The ionic migration-preventing electrode 117 that is also made mainly ofplatinum is disposed between the second and third alumina layers 112,113 and co-fired therewith. One end of the ionic migration-preventingelectrode 117 is electrically connected to the outer terminal pad 156(−)of negative polarity, but the other end thereof is not connected to anyterminal. The ionic migration-preventing electrode 117 has a meanderingline, but it can be straight.

When two alumina layers are used, the heating resistor 115, the leads116 thereof and the migration-preventing electrode 117 may be disposedbetween the first and second alumina layers 111, 112 so that the thirdalumina layer 113 may be eliminated.

Notably, two types of gas sensors that do not cause a serious problemrelating to metal-ion migration are provided according to the presentinvention; one type requires a high-purity alumina ceramic with thepurity thereof being more than 99%, or more preferably more than 99.9%,for use in the alumina substrate, and the other type requires an ionicmigration prevention electrode regardless of the purity of the aluminaceramic constituting the alumina substrate.

The heating resistor 115 embedded in the alumina substrate 11 isnecessary for all types of the gas sensors and a combination thereof. Asunderstood from FIG. 3, a meandering line made mainly of platinum forthe heating resistor 115 and two straight leads thereof 116 that have awider line width than the meandering line are embedded in the aluminasubstrate 11 and are sandwiched by the first and second alumina layers111, 112. One end of each the leads 116 is connected to an end of themeandering line of the heating resistor 115 positioned close to an endof the rectangular-shaped alumina substrate 11. The other end of eachthe leads 116 is extended to two through-holes formed close to the otherends of the second and third alumina layers 112 and 113, and iselectrically connected to either the negative and positive terminal pads156(−), 157(+) that are formed on an outer surface of the third aluminalayer 113. Electric power of 13–14 d.c.v. is normally applied across theterminal pads 156 (−), 157(+) so that the heating resistor 115 generatesheat or thermal energy. The electrical resistance of the heatingresistance is preferably designed to be about 3 Ω.

An ionic migration-preventing electrode may be optionally incorporatedin the laminate (1 a) according to one aspect of the invention. A linemade of mainly of platinum for the ionic migration-preventing electrode117 is formed between the second and third alumina layers 112 and 113,by straightly wiring from a negative outer pad 156 (−) along the lead116 with negative polarity of the heating resistor 115, meanderingthrough an area where the corresponding meandering line of the heatingresistor 115 is correspondingly formed and insulated by the secondalumina layer 112 from the migration preventing electrode 115,straightly wiring along the lead 116 with positive polarity of theheating resistor 115 and ending at a corresponding middle position ofthe lead 116 of positive polarity.

The entire line of the ionic-preventing electrode 117 is charged withnegative polarity, since it is electrically connected only to the outerterminal 156(−) of negative polarity. As a result, metal ions that areelectrically positive are drawn to the ionic-preventing electrode 117 ofnegative polarity.

Specifically, the electric potential of the line of the ionic preventingelectrode may be designed to be equal to or lower than that of the lineof the heating resistor that is comparatively narrow in width andvulnerable. Practically, the ionic migration-preventing electrode 117 isconnected to a portion of the leads 116 of negative polarity, theportion being lower than the heating portion in electric potential.Metallic ions such as Na and K, and Ca, Ba and Mg having a valence oftwo or three and normally forming oxides as an insulating inorganicbinder for alumina grains or contaminants at a low temperature becomechemically unstable in a high temperature exhaust gas ambient. Thesespecies migrate to seek a chemically stable position with respect totheir chemical equilibrium balanced by electric potential, temperature,etc., and move to separate from the oxygen of the oxide in a hightemperature ambient. Instead of the heating resistor 115, the ionicmigration-preventing electrode provides such a stable position for metalions to gather, and at which position the migrated metal ions can bindwith oxygen from the ambient gas to form a new glassy phase when theambient gas temperature is cooling down, since they do not return totheir original position.

Without the migration-preventing electrode, the migrated metal ionscause various serious problems such as electrically disconnecting theheating resistor 115 by volume increase of the glassy phase around theresistor 15 and weakening the alumina substrate 11.

The ionic migration-preventing electrode 117 gathers the migrating metalions instead, thereby vicariously protecting the heating resistor 115from the metal ions migrating toward the heating resistor 115 so thatthe glassy phase causing the various problems is not formed around thevulnerable heating resistor 115. This is the reason why the electricpotential of the ionic migration-preventing electrode should bemaintained equal to or lower than that of the heating resistor in anaspect of the invention. In other words, the electrical potential of theionic migration-preventing electrode 117 should be equal to or lowerthan the electric potential at a position connecting the heatingresistor 115 and leads 116 of the heating resistor 115.

In another aspect of the invention, the position of the ionicmigration-preventing electrode also becomes important, if the ionicmigration-preventing electrode is used in a laminate (1 a). This isbecause a new glassy phase is inevitably formed around the ionicmigration-prevention electrode. Since the new glassy phase weakens thestrength of the alumina ceramic surrounding the migration-preventionelectrode, the position where the glassy phase can be allowed to formneeds to be carefully designed, from the view-points of reliability,endurance, size compactness, heating efficiency of the heating resistorand activation of the sensor cells.

In an aspect of the invention, the ionic-migration-prevention electrode117 for preventing the heating resistor 115 from degradation orelectrical disconnection is formed or disposed, not between the heatingresistor 115 and the electrode 136 of the oxygen-detecting cell 13 (orany electrode 133, 126, 123, regardless of the position of the cells 12,13). Since the electric potential of the migration-preventing electrode117 is lower than that of the heating resistor 115, the metal ionsmigrate toward the migration-prevention electrode 117 and form theglassy phase around the ionic migration-preventing electrode 117. Inother words, a new glassy phase or new glass formed by migrated metalions around the migration-prevention electrode 117 should not be formedbetween the heating resistor 115 and any electrode of cells 12, 13.

In the case that the migration-prevention electrode and the heatingresistor is formed on the same plane, for instance, as sandwiched by thefirst and second alumina layers 111, 112, an electric line of theheating resistor should run far away as possible from the electrode 136of the oxygen-detecting cell 13 and a line of the heating resistor suchthat the new glassy phase is formed closer to an outer surface of thealumina substrate 11 than to the electrode 136 of the oxygen detectingcell 13. In this manner, both the cell electrode 136 and the heatingresistor are protected from the new glassy phase.

Platinum or platinum-containing material is used for these electricalwirings of the ionic migration prevention electrode 117 and the heatingresistor 115 including the leads 116 thereof, and for those electrodes133, 136, 123, 126 of the oxygen-detecting cell 13 and theoxygen-pumping cell 12 including the leads 134, 137, 124, 127, and forthe outer terminal pads 153, 154, 155, 156. This is because platinum canbe co-fired in an oxidizing atmosphere in which the alumina substrateand the zirconia layers are co-fired, for instance at a temperature ofabout 1450–1560 degrees centigrade.

Alumina-containing material is used at least for the alumina substrate11, the ion-leakage prevention spacer 143, the reduction-preventinglayers 128, 129, 138 and the oxygen-ion conductive solid electrolytelayers 121, 131. This is because improved quick activation of theoxygen-detecting cell 13 and the oxygen-pumping cell 12 is attained.

The alumina layers 111, 112, 113 constituting the alumina substrate 11,may contain up to 30% by weight of a heterogeneous metal oxide materialsuch as silica, magnesia and calcia, other than alumina, or may bebonded by firing with such a heterogeneous material, so long as theionic migration-preventing electrode 117 is incorporated in the aluminasubstrate 11. However, oxides of alkaline and alkaline earth metalshould be minimized if they have to be added as an inorganic binder intothe alumina substrate 11.

The alumina content may be varied or different from each other among thefirst, second and third alumina layers 111, 112, 113 from view points ofheating efficiency by the heating resistor 115 for the cells 12, 13,prevention of metal ions from migrating through the alumina substrate 11and/or co-firing conditions with the cell electrolyte layer 131. Atypical alumina substrate containing 4% by weight of silica, 3% byweight of MgO and 1% by weight of calcia needs theionic-migration-preventing electrode 117.

When the content of the alumina in the first and second alumina layers111, 112 that sandwich or surround the heating resistor 115 is more than99% by weight, or preferably more than 99.9% by weight (in other words,when the content of the heterogeneous metal oxide material contained inthe alumina layers 111, 112 other than alumina is not more than 1% byweight, or preferably not more than 0.1% by weight), the ionic migrationof metal ions is greatly decreased. As a result, the ionicmigration-preventing electrode for preventing the heating resistor 115from degradation or electrical disconnection may be eliminated. Mostpreferably, when the heterogeneous material contained in the aluminalayers 112, 113 is less than 0.01% by weight, the ionic migration iscompletely shut out and the ionic migration-preventing electrode 117 isunnecessary.

Referring to FIG. 3, a portion of the first oxygen-ion conductive solidelectrolyte layer 131 that is co-fired with the alumina substrate 11constitutes the oxygen-detecting cell 13. The first oxygen-ionconductive solid electrolyte layer 131 is of a rectangular shape with athrough-hole formed near a corner formed by a longitudinal side end anddistal end thereof.

The lead 137 formed extendedly from an oxygen-reference electrode 136and closely along the longitudinal side of the first oxygen-ionconductive solid electrolyte layer 131 is further extended so as toelectrically connect to a first outer pad 155 formed on a surface of areinforcing insulative cover layer 152 via a through-hole penetratingthe first and second solid electrolyte layers 131, 121,reduction-preventing insulative layers 138, 128, 129, an ion leakagepreventing ceramic spacer 143 and the reinforcing insulative layer 152.

The oxygen-detecting electrode 133 of the oxygen-detecting cell 13 isformed and co-fired between the first oxygen-in conductive solidelectrolyte layer 131 and the first alumina layer 111. The lead 134connecting one end thereof to the oxygen-detecting electrode 133 andconnecting the other end thereof to a second outer terminal 154 iscofired between a reduction-preventing insulative layer 138 and anion-leakage preventing ceramic spacer 143. The lead 134 runs along alongitudinal center of the first oxygen-ion conductive solid electrolytelayer 131 to a distal end portion of the first electrolyte layer 131 soas to be connected electrically the second outer pad 154 formed on thereinforcing insulative layer 152 via a through-hole penetrating theion-leakage preventing ceramic spacer 143, first and secondreduction-preventing insulative layers 128, 129 and a reinforcinginsulative layer 152.

The oxygen-reference electrode 136 and an oxygen-detecting electrode 133are cofired on different faces of the oxygen-ion conductive solidelectrolyte layer 131, respectively, so as to constitute theoxygen-detecting cell 13. The oxygen-detecting electrode 133 and theoxygen-reference electrode 137 of the oxygen-detecting electrode 13 arepositioned to correspond to the position of the heating resistor 115that in turn is positioned beneath the first alumina layer 111.

The first oxygen-ion conductive solid electrolyte layer 131 interfaceswith the first alumina substrate 111, the oxygen-reference electrode 136and the lead 137 thereof, and also interfaces with the oxygen-detectingelectrode 133 of the oxygen-detecting cell 13 and thereduction-preventing insulative layer 138.

The first oxygen-ion conductive solid electrolyte layer 131 containszirconia and alumina. Preferably the amount of alumina in the firstoxygen-ion conductive solid electrolyte layer 131 is 10–80% by weight,and more preferably 20–60% by weight, based on 100% by weight of theentire first solid electrolyte layer.

Notably, high purity alumna of more than 99% purity or preferably 99.9%purity with a particle size of less than 1 micrometers should becontained at least in the second oxygen-ion conductive-solid electrolytelayer 121 for the oxygen-pumping cell 12 and optionally, yet preferablyin the oxygen-ion conductive solid electrolyte layer 131 for theoxygen-detecting cell 13. This is from a view point of lowering internalresistance of the oxygen-pumping cell 12 and control of zirconia phaseso as not to transform to monoclinic phase.

The zirconia used in the oxygen-solid electrolyte layer 121 laminated inthe laminate (1 a) is a partially stabilized zirconia substantiallyconsisting of tetragonal phase and cubic phase with substantially nomonoclinic phase. The finer the average grain size of alumina grainsthat is used, the better structural strength and stability against phasetransformation from tetragonal to monoclinic is attained in a thermalcycling under high humidity, when the average grain size of zirconia isless than 2.5 micrometers, as measured by a known method using ascanning electron microscope (SEM) such as that described in EP1026502A2. This is because fine alumina grains existing at grainboundaries of zirconia suppress grain growth and phase-transformation ofzirconia.

The zirconia included in the first and second oxygen-ion conductivesolid electrolyte layers 131, 121 is a partially stabilized zirconia,for instance, partially stabilized by 2–9 mol % of yttria. Preferably, apartially stabilized zirconia with its phase (or phases) substantiallyconsisting of tetragonal and cubic phases, containing no monoclinicphase or rather less than 5% by weight, is used in the first and secondoxygen-ion conductive solid electrolyte layers 131, 132.

A preferable phase ratio of cubic phase to tetragonal phase formed inzirconia of the solid electrolyte layers 131, 121 is from 1:4 to 2:1,more preferably from 1:3 to 3:1, or most preferably 3:7 to 1:1. Thisphase ratio may be varied, based dependently on the amount of thealumina included in the zirconia electrolyte layers 131, 121. The phaseratio is determined by a known X-ray diffraction analysis, specificallybased on comparison of X-ray diffraction peak intensities on monoclinic,tetragonal and/or cubic phases between samples of known values andspecimens to be analyzed, detected on crystal face (400) for cubic phaseand crystal faces (004) and (220) for tetragonal phase.

So long as this phase ratio falls in the above range of from 1:4 to 2:1with inclusion of alumina in the zirconia used in the second oxygen-ionconductive solid electrolyte layer 121 of the oxygen-pumping cell 121and with substantially no monolithic phase that causes a volume changeof zirconia (due to phase transformation) leading to cracks of the solidelectrolyte layer 121, a sensor having a laminate (1 a) shows gooddurability and structural strength and does not malfunction in a thermalcycling environment.

An ion-leakage preventing ceramic spacer 143 made mainly of aluminaforms a diffusion space 141. The ion-leakage preventing spacer 143 is,as shown in FIG. 3, laminated and co-fired with the first oxygen-ionconductive solid electrolyte layer 131, the lead 134 of theoxygen-detecting cell 12 and the reduction-prevention insulative layer138 interposed between the lead 134 and the first solid electrolytelayer 131. When the thickness of the ion-leakage preventing ceramicspacer 143 is decreased, as shown in FIG. 6, the thickness of the firstand/or second solid electrolyte layers 131, 121 may be increased so asto ensure a minimum gas-diffusion space 141 between an oxygen-detectingelectrode 133 of the oxygen-detecting cell 13 and an inner electrode 126of the oxygen-pumping electrode 12.

The ion-leakage preventing ceramic spacer 143 has a notched portion nearor at one of its distal ends so as to form the diffusion space 141 andto incorporate therein a gas diffusion passage 142 filled with a porousinsulative alumina ceramic. The ion-leakage preventing ceramic spacer143 is so shaped to prevent oxygen-ions of a second oxygen-ionconductive solid electrolyte layer 132 from leaking to the firstoxygen-ion conductive solid electrolyte layer 121, in co-operation withthe diffusion passage 142 filled with the porous insulative ceramicmaterial.

Since the ion-leakage prevention spacer is made mainly of alumina, itshigh thermal conductivity is advantageous in minimizing a temperaturedifference between the first and second solid electrolyte layers 121,131 and in transferring a heating energy from the heating resistor 115to the second solid electrolyte layer 121. Without such an ion-leakagepreventing ceramic spacer 143, when the oxygen-pumping cell 12 pumpsoxygen from or into the diffusion space 141, the second oxygen-ionconductive layer 12 leaks or floods its oxygen-ions into the firstoxygen-ion conductive layer 131, causing erroneous detection ofelectromotive force generated across the oxygen-detecting electrodes 133and the oxygen-reference electrode 136 of the oxygen-detecting cell 13and resulting in erroneous measurement of ionic current that flowsacross the oxygen-pumping cell 12. Therefore, this ion-leakageprevention ceramic spacer 143 is necessarily laminated directly orindirectly with the first oxygen-ion conductive solid electrolyte layer131 to provide accurate detection or sensing of oxygen amount (or oxygenconcentration) by the oxygen-detecting cell 13.

Notably, in order to further prevent erroneous measurement or otheradverse effects of ion-leakage, at least one electrically conductiveline running via through-holes penetrating the first and/or secondoxygen-ion conductive solid electrolyte layers 131, 121 may be insulatedfrom the first and/or oxygen-ion conductive solid electrolyte layers bya circumferential insulative layer 130 of alumina inside thethrough-holes, as shown in FIG. 2.

An electromotive force detected across the electrodes 133, 136 isrelated to the ratio between an oxygen-partial pressure of the gasatmosphere inside the gas diffusion space 141 and that of anoxygen-reference gas, according to the Nernst equation. Therefore, avoltage detected across the oxygen-detecting electrode 133 and theoxygen-reference electrode 136 is indicative of oxygen partial pressureinside the gas diffusion space 141.

A second oxygen-ion conductive solid electrolyte layer 121 is laminatedand co-fired with the oxygen-ion leakage preventing ceramic spacer 143that is laminated and co-fired with the first oxygen-ion conductivesolid electrolyte layers 131 that is laminated and co-fired with thealumina substrate 11. The second oxygen-ion conductive solid electrolytelayer 121 is indirectly co-fired with the alumna substrate 11 throughthe first oxygen-ion conductive solid electrolyte layer 121 interveningtherebetween.

A portion of the second-ion conductive solid electrolyte layer 121constitutes the oxygen-pumping cell 12 with outer and inner electrodes123, 126 formed on different respective faces of the second-ionconductive solid electrolyte layer 121. The second oxygen-ion conductivesolid electrolyte layer 121 is of a rectangular shape with twothrough-holes; one is formed near a corner formed by its one distal endand longitudinal side end, while the other is formed close to a middleof the distal end. The through-hole formed near the corner is used forelectrical connection for the lead 137 of the oxygen-detecting cell 131.

The lead 127 that is formed extendedly from an inner electrode 126 ofthe second-ion conductive solid electrolyte layer 121 and furtherextended along about a longitudinal center of an ion-leakage-preventionceramic spacer 143, is electrically connected to the second outer pad154 formed on a surface of the reinforcing insulative cover layer 152via a through-hole penetrating the first and second reduction-preventinginsulative layers 128, 129, the second solid electrolyte layer 121, andthe reinforcing insulative ceramic cover 152, and is electricallyconnected with the lead 134 of the oxygen-detecting electrode 133 of theof the oxygen-detecting cell 13 as shown in FIGS. 2 and 3, so that theinner electrode 126 of the pumping electrode 12 and the oxygen-detectingelectrode 133 of the oxygen-detecting cell 13 can be electricallycharged with the same electric potential.

A first reduction-preventing insulative layer 128 is interposed betweenthe lead 127 and the second oxygen-ion conductive solid electrolytelayer 121, for preventing so called “blackening” or rather reduction ordeoxidization of zirconia contained in the second oxygen-ion layer 121surrounding the lead 127.

A lead 124 is electrically connected to the third outer pad 153 formedon the surface of the reinforcing insulative ceramic layer 152, asunderstood by FIG. 3, via a through-hole penetrating the reinforcinginsulative ceramic cover 152 and is also electrically connected to theouter electrode 123 of the oxygen-pumping cell 12. The lead 124 isformed and co-fired between the reinforcing insulative cover 152 and thesecond reduction-preventing insulative layer 129 that preventsblackening or rather reduction or deoxidization of zirconia contained inthe second solid electrolyte layer 121 surrounding the lead 124. Thelead 124 runs near along a longitudinal side of the second oxygen-ionconductive solid electrolyte layer 121 and from the outer electrode 123formed near a distal end portion of the second solid electrolyte layer112 to the other distal end portion where the two through-holes areformed through the oxygen-ion conductive solid electrolyte layer 121.

The inner and outer electrodes 126, 123 of the oxygen-pumping cell 12are positioned to correspond to the position of the oxygen-detectingelectrode 133 of the oxygen-detecting cell 13, which in turn ispositioned apart and spaced by the gas-diffusion space 141 from theinner electrode 126. The second oxygen-ion conductive solid electrolytelayer 121 interfaces with the inner electrode 126 of the oxygen-ionpumping cell 12, the ion leakage preventing ceramic spacer 143 and thefirst reduction-preventing insulative layer 128. The oxygen-ionconductive-solid electrolyte layer 121 also interfaces with the outerelectrode 123 of the oxygen-ion pumping cell 12 and the secondreduction-preventing insulative layer 129.

The second reduction-preventing insulative layer 129 has a large hole ofa rectangular shape at its distal end portion, in which hole the outerelectrode 123 is formed on the oxygen-ion conductive solid electrolytelayer 121.

A gas diffusion space 141 is formed between an inner electrode 126 ofthe oxygen-pumping cell 12 and the oxygen-detecting electrode 133 of theoxygen-detecting cell and formed by the oxygen-ion leakage preventionspacer 143 and a gas-diffusion passage 142 filled with a porous aluminaceramic. The porous alumina ceramic renders a diffusion resistanceagainst gas molecules entering the gas diffusion space 141, and preventsoxygen ion leakage across the two cells 12, 13.

The gas-diffusion passage 142 is formed between ends of the first andsecond oxygen-ion conductive solid electrolyte layers 131,121. Thegas-diffusion passage 142 is filled with a porous insulative ceramicmaterial such as porous alumina having a diffusion resistance thatphysically limits the amount of molecules of gaseous components enteringor rather diffusing into the gas diffusion space 141. Since the gaseousmolecules including oxygen are limited by the diffusion resistance ofthe gas-diffusion passage 142, an amount of oxygen inside thegas-diffusion space 141 can be controlled to a constant target value bypumping oxygen in or out of the gas-diffusion space 141. Feedbackcontrol is effected through a control circuit by comparing an oxygenpartial pressure value detected inside the gas-diffusion space 142 withthe target value and then electrically ordering the oxygen-pumping cellto pump out oxygen (if the oxygen is abundant inside the diffusionspace) or to pump in oxygen (if the oxygen is scarce therein) withproper polarity of voltage applied across the inner and outer electrodes123, 126 of the oxygen-pumping cell 12 so long as there is a differencebetween the two values. In this manner, oxygen is ionized to flowthrough the oxygen-ion conductive layer 121 sandwiched by the electrodes121, 123 of the oxygen-pumping cell 12. The ionized oxygen flowingacross the electrodes 121, 123 can be electrically measured as an ioniccurrent of the oxygen-pumping cell. This ionic current can be used as anindicator of a state of oxygen amount or partial oxygen pressure of theambient gas such as an exhaust gas of an internal combustion engine,covering a wide range of air fuel ratio from fuel-lean to fuel-rich,when an entering speed and an out-going speed of the oxygen inside thegas-diffusion space 141 are well balanced by design.

As shown in FIG. 7, two gas-diffusion passages 142 each with a diffusionresistance may be formed between sides of the oxygen-ion conductivelayers 131 and 132. Such a gas-diffusion passage 142 may be formedalternatively as shown in FIG. 4, penetrating an oxygen-pumping cell 12and a porous protective cover 151 covering an outer electrode of thepumping cell 12 or further alternatively as shown in FIG. 5, penetratingan oxygen-detecting cell 13 and an alumina substrate 11 in which aheating resistor 115 and/or an ionic migration prevention preventingelectrode 117 are embedded. Among these gas-diffusion passage choices,plural gas-diffusion passages formed as shown in FIG. 7 become mostadvantageous. Namely, adjustment of diffusion resistance becomes easybecause of the plural passages each filled with a porous insulativematerial.

Referring back to FIG. 3, the reduction-preventing insulative layer 138mainly made of alumina provided between the lead 134 of theoxygen-detecting cell 13 and the solid electrolyte layer 131, isco-fired therewith. Although the lead 134 thereof is not so seriouslyreduced or deoxidized, accurate detection of electromotive forcegenerated across the detecting electrode 133 and the reference electrode136 of the oxygen-detecting cell 13 is effectively improved, compared toa sensor laminate without the insulative layer 138.

Two other reduction-preventing insulative layers 128, 129 made mainly ofalumina are importantly provided and cofired between the secondoxygen-ion conductive solid electrolyte layer 121 and the leads 124, 128respectively. These reduction-preventing insulative layers 128, 129protect zirconia of the oxygen-ion conductive solid electrolyte layer121 surrounding the leads from blackening (meaning deoxidization orreduction). This “blackening” phenomenon liable to occur without theinsulative layers 128, 129 is caused by a voltage applied across theleads 124 of an outer electrode 123 and an inner electrode 126 of theoxygen-pumping cell 12 for pumping oxygen into or out of thegas-diffusion space 141. Specifically, the “blackening” occurs aroundthe electrode 127 in the fuel-lean state and it occurs around theelectrode 124 in the fuel-lean state, depending on the applied voltagepolarity subjected thereto.

A reinforcing insulative cover 152 mainly made of alumina is co-fired onthe second oxygen ion-conductive layer 121 and the lead 124 of theoxygen-pumping cell 12 so as to protect them electrically and tomechanically increase the overall strength of the laminate (1 a).

A porous insulative cover 151 is formed on the outer electrode 123entirely covering it. A preferable material for this cover 151 is, forinstance, porous alumina or porous spinel. The porous insulative cover151 prevents the outer electrode 123 of the pumping cell 12 fromcontaminants such as water, dirt including Si, Pb, P, etc. contained inambient exhaust gas from an internal combustion engine.

Dimensional and functional optimization is inevitable in designing thelaminate (1 a) and selecting materials for the laminated componentsthereof. With respect to the overall dimension of the laminate (1 a) fora gas sensor such as a full range air-fuel ratio sensor for use incontrolling an automobile combustion engine, the longitudinal length,width, and thickness of the laminate should be optimized within 30–60mm, 3–6 mm, and 1–3 mm, respectively. If the length is less than 30 mm,the area where the outer terminal pads 153, 154, 155 becomes too hot andthe electrical insulation between the pads is lost under a practicalhigh temperature environment.

Specifically, the thickness of the first oxygen-ion conductive solidelectrolyte layer 131 that is closely laminated on the alumina substrate11 embedding the heating resistor 115 is optimized in the range of10–200, preferably 20–100 and more preferably 30–70 in micrometers. Ifit is less than 10 micrometers, sufficient electromotive force orvoltage is not generated across the oxygen-detecting cell 13. If it ismore than 200 micrometers, heating efficiency for the oxygen-pumpingcell 12 by the heating resistor is greatly affected.

With respect to the thickness of the second oxygen-ion conductive solidelectrolyte layer 121, the minimum thickness is increased to 30micrometers, compared to that of the first solid electrolyte layer 131.This is because a portion of the solid electrolyte layer 121 requiredfor an oxygen-pumping function of the oxygen-pumping cell 12 issuspended over the gas diffusion space 141 and not supported by a sturdysubstrate, except by the outer electrode 123 and the porous insulativecover 151 which are not sturdy, compared to the alumina substrate 11 bywhich the first solid electrolyte layer 131 is entirely supported.However the maximum thickness of the second solid electrolyte layer 121may not exceed 400 micrometers, primarily because the oxygen-pumpingcapability of the oxygen-pumping cell 12 is greatly affected due tolimitation of allowable area for its inner and outer electrodes, whicharea is limited by the overall dimension of the laminate (1 a)acceptable in a gas sensor housing as mentioned above. In addition,activation of the oxygen-pumping cell 12 is unimproved, even if aluminagrains are added in the first and second solid electrolyte layer 131,121 due to a volume increase of the suspended portion of the secondsolid electrolyte layer 121 for the oxygen-pumping cell 12 to be heated.

Notably, the suspended portion of the second solid electrolyte 121 isnot sharply heated due to the diffusion gas space 141 blocking directthermal transfer of heat generated by the heating resistor 115. This isadvantageous for the gas sensor with the co-fired laminate (1 a), sincethe oxygen-pumping cell 12 is protected or buffered from a thermal shockrendered by the heating resistor 115.

The thickness of the electrodes of the sensor cells 12, 13, ispreferably in the range of about 3 to 30 micrometer (μm), or morepreferably 10–25 μm. The area of the electrode 126 of the oxygen-pumpingcell is preferably in the range of about 1–20 mm², or more preferably6–10 mm². The area of the electrode 133 of the oxygen-detecting cell 13is preferably 15–80% of that of the electrode 126 of the oxygen-pumpingcell 12.

Inclusion of alumina grains into a solid electrolyte layer formed ofpartially stabilized zirconia suppress not only phase transformation butalso grain growth of zirconia effectively, depending on the amount andaverage grain size of the alumina grains.

Suppression of the phase transformation and grain growth of zirconiabecomes more effective when the included alumina grains are in an amountof 10–80% by weight and with an average grain size of less than 1 μm.Notably, since the alumina grains are not ion-conductive, purifiedalumina containing substantially no contaminant or less than 0.01% byweight of contaminant is most preferably recommended for a green(unfired) layer for the oxygen-ion conductive solid electrolyte layer121 constituting the oxygen-pumping cell 12. The silica contentespecially should be substantially zero, because it damages the oxygenion conductivity of the co-fired zirconia solid electrolyte layers 121,131.

The average grain size of alumina powder to be contained in the green(unfired) oxygen-ion conductive solid electrolyte layers 121, 131 ispreferably in the range of 0.1–0.5 micrometers. The average grain sizeof zirconia powder to be contained in a green layer for the oxygen-ionconductive layers 121, 131 is preferably in the range of 0.2–1.2micrometers. The use of a co-precipitated zirconia powder containing 3–7mol % yttria serving as a stabilizer is recommended.

When the solid electrolytic layer after co-firing contains aluminahaving an average grain size as mentioned above, the average grain sizeof YSZ (yttria stabilized zirconia) after co-firing can be restrained toless than 2.5 μm. Since such YSZ containing alumina is used in thesensor laminate (1 a), the phase transformation caused by a temperaturerise and a temperature drop during a cold-hot thermal cycle in an actualenvironment in which the sensor laminate (1 a) is used as a gas sensoris restrained very effectively. This is probably because a stress causedby the phase transformation is readily dispersed or absorbed by the finealumina grains abundantly existing at zirconia grain boundaries suchthat the occurrence of cracks is prevented.

In production of the laminate (1 a) that does not have an ionicmigration-preventing electrode 117, highly purified alumina powder withcontaminants of less than 1% by weight or substantially no contaminants(preferably of less than 0.1% by weight or most preferably less than0.01% by weight) is used for preparing green layers for the first andsecond alumina layers 111, 112 surrounding the heating resistor 115 andfor the first and second oxygen-ion conductive solid electrolyte layers113, 121. In preparing these green layers 113, 121 and/or other greenlayers 128, 129, 138, 143, 152 for co-firing the laminate (1 a), a knownmethod such as a doctor blade method and a sheet-rolling method may beapplied. A screen-printing method may be used for printing unfiredelectrodes 133, 136, 123, 126 and/or unfired leads thereof 134, 137,124, 127 onto the green first and second solid electrolyte layers 131,121 and/or other green layers 127, 129, 143, for printing unfiredheating resistor 115 and leads thereof 116 onto unfired second aluminalayer 112, for printing unfired outer terminal pads 157(+), 156(−) ontogreen second alumina layer 112 (or 113), and for printing unfired first,second and third outer terminal pads 155, 154, 153 onto greenreinforcing insulative cover 152 layer. The screen-printing method isalso applicable in forming the porous protective cover 151 and theporous material filled in the gas-diffusion passage 142. Notably, amaterial for the porous protective cover 151 and the porous material tobe filled in the gas-diffusion passage 142 are made by mixing 30–70% byvolume of alumina powder and 70–30% by volume of carbon powder andco-firing with other laminated components.

In production of the laminate (1 a) that has a migration-preventingelectrode 117 embedded in the alumina substrate 11, similar methods asdescribed above are applicable, except that metal oxides such as MgO,CaO, BaO, SiO₂ may be used as inorganic binders in green aluminasubstrate 11 comprising green first, second and third alumina layers111, 112, 113 and that an unfired migration-preventing electrode 117 isprinted on the unfired third alumina layer 113.

After laminating these unfired components containing necessary organicbinders to form an unfired laminate (1 a), the unfired laminate (1 a) issimultaneously sintered (i.e., co-fired) under an optimized firingschedule, for instance, of gradually elevating the temperature by 10degrees centigrade per hour up to 420 degrees centigrade and holding fortwo hours at that temperature, then elevating by 100 degrees centigradeper hour up to 1100 degrees centigrade and further elevating by 60degrees centigrade per hour up to 1520 degrees centigrade at whichtemperature holding for one hour, and then cooling so as to attain aco-fired gas sensor laminate (1 a) that operates as a full rangeair/fuel ratio sensor (or so called UEGO: universal exhaust gas oxygensensor).

In operation of the sensor laminate (1 a) thus attained, firstly a dcvoltage of about 13 V is applied across the terminal pads 156(−), 157(+)so as to heat and activate the sensor cells 12, 13. Then a negligiblylittle constant current of about 10 microampere is flowed across theoxygen-detecting electrode 133 and the reference electrode 136 of theoxygen detecting cell 12 through an resistor so that the referenceelectrode 136 maintains a constant oxygen partial pressure of about 2atm for use as a reference. Excess oxygen in the reference electrodedrains through the lead 137 thereof that is porous. The oxygen-detectingelectrode 133 now determines an oxygen partial pressure inside the gasdiffusion space 141 because the reference electrode 136 holds a constantreferable oxygen partial pressure. At this moment, when the oxygenpartial pressure inside the gas diffusion space 141 is not apredetermined value (corresponding to, e.g., stoichiometric air fuelratio λ), a controller electrically orders the oxygen-pumping cell 12 topump oxygen in or out of the gas diffusion space 141 until theoxygen-detecting cell 13 detects the predetermined value (of about 450mV corresponding to λ). Therefore, as is well known, an ionic pumpingcurrent flowing across the electrodes 123, 126 of the oxygen-pumpingcell 12 becomes indicating an extent of oxygen amount away from thepredetermined value (corresponding to λ) and referring to a burningstate of the fuel in an internal combustion engine. Based on this ioniccurrent, the internal combustion engine can control an air-fuel ratiofrom fuel-lean to rich and/or a burning condition of the fuel in theengine. Additional functional details of the UEGO are described in “TheFundamentals of Automotive Engine Control Sensors, authored by KanemitsuNishio, published by Fontis Media (Switzerland, 2001).

The gas sensor having the sensor laminate (1 a) according to theinvention survives a 500-hour thermal cycling durability test conductedon an engine dyno fixture in which the temperature reversibly variesbetween 350 to 930 degrees centigrade in a hour under A/F ratio shiftingof 12 to 30.

The present invention is not limited to the above-described embodiments.Numerous modifications and variations in the present invention arepossible according to purpose or application without departing from thescope of the invention.

This application is based on Japanese Patent Application No. 2002-320479filed Nov. 1, 2002, the disclosure of which is incorporated herein byreference in its entirety.

1. A gas sensor (1 a) having a laminate comprising: an alumina substrate(11) having a heating resistor (115) embedded in the alumina substrate(11); a first oxygen-ion conductive solid electrolyte layer (131)containing zirconia and alumina and partly constituting anoxygen-detecting cell (13) and said first solid electrolyte layer (131)being laminated with said alumina substrate (11); a second oxygen-ionconductive solid electrolyte layer (121) containing zirconia and aluminaand partly constituting an oxygen-pumping cell (12); an ion-leakagepreventing ceramic spacer (143) for preventing oxygen-ions from leakingfrom the second oxygen-ion conductive solid electrolyte layer (121) tothe first oxygen-ion conductive solid electrolyte layer (131), saidspacer (143) being laminated between said first and second oxygen-ionconductive solid electrolyte layers (131, 121); and a gas-diffusionspace (141) formed between an electrode (133) of the oxygen-detectingcell (13) and an electrode (126) of the oxygen-pumping cell (12);wherein the first and second oxygen-ion conductive solid electrolytelayers contain alumina grains, and wherein the second oxygen-ionconductive solid electrolyte layer (121) contains alumina in an amountless than that of the first oxygen-ion conductive solid electrolytelayer (131).
 2. The gas sensor as claimed in claim 1, wherein thelaminate is a co-fired laminate.
 3. The gas sensor as claimed in claim1, further comprising an ionic migration-preventing electrode (117) forpreventing the heating resistor (115) from deterioration or electricaldisconnection, wherein an electric potential of said ionicmigration-preventing electrode (117) is equal to or lower than thelowest electric potential of any part of the heating resistor (115). 4.The gas sensor as claimed in claim 3, wherein said ionicmigration-preventing electrode (117) is formed between an outer surfaceof the alumina substrate (11) and the heating resistor (115).
 5. The gassensor as claimed in claim 4, wherein the first and second oxygen-ionconductive solid electrolyte layers (131, 121) contain 10–80% by weightof alumina, respectively, and an average grain size of alumina containedin the first and/or second oxygen-ion conductive solid electrolytelayers (131, 121) is less than 1 micrometer.
 6. The gas sensor asclaimed in claim 4, wherein a phase of the zirconia contained in thefirst and/or second oxygen-ion conductive solid electrolyte layerconsists essentially of a cubic phase and a tetragonal phase.
 7. The gassensor as claimed in claim 4, wherein the second oxygen-ion conductivesolid electrolyte layer (121) contains alumina in an amount that is atleast 5% by weight less than that of the first oxygen-ion conductiveelectrolyte layer (131).
 8. The gas sensor as claimed in claim 3,wherein said ionic migration-preventing electrode (117) is connected toa portion of leads (116) of negative polarity, said portion being lowerthan the heating resistor (115) in electric potential.
 9. The gas sensoras claimed in claim 3, wherein the first and second oxygen-ionconductive solid electrolyte layers (131, 121) contain 10–80% by weightof alumina, respectively, and an average grain size of alumina containedin the first and/or second oxygen-ion conductive solid electrolytelayers (131, 121) is less than 1 micrometer.
 10. The gas sensor asclaimed in claim 3, wherein a phase of the zirconia contained in thefirst and/or second oxygen-ion conductive solid electrolyte layersubstantially consists of a cubic phase and a tetragonal phase.
 11. Thegas sensor as claimed in claim 3, wherein the second oxygen-ionconductive solid electrolyte layer (121) contains alumina in an amountthat is at least 5% by weight less than that of the first oxygen-ionconductive electrolyte layer (131).
 12. The gas sensor as claimed inclaim 1, wherein the second oxygen-ion conductive solid electrolytelayer (121) constituting the oxygen-pumping cell (12) contains 10–80% byweight of alumina.
 13. The gas sensor as claimed in claim 1, wherein thefirst oxygen-ion conductive solid electrolyte layer (131) constitutingthe oxygen-concentration cell (13) contains 10–80% by weight of alumina.14. The gas sensor as claimed in claim 1, wherein the first and secondoxygen-ion conductive solid electrolyte layers (131, 121) contain 10–80%by weight of alumina, respectively, and an average grain size of aluminacontained in the first and/or second oxygen-ion conductive solidelectrolyte layers (131, 121) is less than 1 micrometer.
 15. The gassensor as claimed in claim 14, wherein a phase of the zirconia containedin the first and/or second oxygen-ion conductive solid electrolyte layersubstantially consists of a tetragonal phase and/or cubic phase.
 16. Thegas sensor as claimed in claim 14, wherein the second oxygen-ionconductive solid electrolyte layer (121) contains alumina in an amountthat is at least 5% by weight less than that of the first oxygen-ionconductive electrolyte layer (131).
 17. The gas sensor as claimed inclaim 14, wherein the second oxygen-ion conductive solid electrolytelayer 121 constituting the oxygen-pumping cell (12) contains 60 to 90%by weight of zirconia and 10–40% by weight of alumina, and wherein thesecond oxygen-ion conductive solid electrolyte layer (121) constitutingthe oxygen-pumping cell (12) contains alumina in an amount that is 10 to50% by weight less than that of the first oxygen-ion conductiveelectrolyte layer (131) constituting the oxygen-detecting cell (13). 18.The gas sensor as claimed in claim 14, further comprising an electrode(136) and a lead (137), wherein the electrode (136) constituting theoxygen-concentration detecting cell (13) and facing the aluminasubstrate (11) is a reference electrode capable of storing oxygentherein and communicating with an atmosphere outside the sensor (1 a)through a lead (137) connected to the electrodes (136), said lead (137)serving as a channel (16) for draining oxygen.
 19. The gas sensor asclaimed in claim 14, wherein an area of the said electrode (133) of theoxygen-detecting cell (13) is 15 to 80% that of the electrode (126) ofthe oxygen-pumping cell (12).
 20. The gas sensor as claimed in claim 1,wherein the second oxygen-ion conductive solid electrolyte layer (121)constituting the oxygen-pumping cell (12) contains 60 to 90% by weightof zirconia and 10–40% by weight of alumina, wherein the firstoxygen-ion conductive solid electrolyte layer (131) constituting theoxygen-detecting cell (13) contains 40 to 80% by weight of zirconia and20–60% by weight of alumina, and wherein the second oxygen-ionconductive solid electrolyte layer (121) constituting the oxygen-pumpingcell (12) contains alumina in an amount that is 10 to 50% by weight lessthan that of the first oxygen-ion conductive electrolyte layer (131)constituting the oxygen-detecting cell (13).
 21. The gas sensor asclaimed in claim 1, further comprising an electrode (136) and a lead(137), wherein the electrode (136) constituting the oxygen-concentrationdetecting cell (13) and facing the alumina substrate (11) is a referenceelectrode capable of storing oxygen therein and communicating with anatmosphere outside the sensor (1 a) through a lead (137) connected tothe electrodes (136), said lead (137) serving as a channel (16) fordraining oxygen.
 22. The gas sensor as claimed in claim 1, furthercomprising a reduction-preventing insulative layer (128) for preventingdeoxidation of the second oxygen-ion conductive layer (121) around thelead (127) of the oxygen-pumping cell (12), said reduction-preventinginsulative layer (128) being provided between a lead (127) of theoxygen-pumping cell (12) and the oxygen-ion conductive solid electrolytelayer (121).
 23. The gas sensor as claimed in claim 1, wherein saidalumina substrate contains at least 99% by weight of alumina.
 24. Thegas sensor as darned in claim 1, further comprising a reinforcinginsulative cover (152) for reinforcing the second oxygen ion-conductivelayer (121) and covering the lead (124) of the oxygen pumping cell (12).25. The gas sensor as claimed in claim 1, wherein a distance between theelectrodes (126, 133) defining the gas diffusion space (141) is 20 to 80micrometers.
 26. The gas sensor as claimed in claim 1, wherein athickness of the oxygen-ion conductive solid electrolyte layer (131)constituting the oxygen-detecting cell (13) is 10–200 micrometers, saidgas sensor further comprising an electrode (136) located between theoxygen-ion conductive solid electrolyte layer (131) and the aluminasubstrate (11) and having a thickness of 1–20 micrometers.
 27. The gassensor as claimed in claim 1, further comprising a gas-diffusion passage(142) through which the measurement gas enters the gas diffusion space(141), said gas-diffusion passage (142) being formed through theion-transfer prevention spacer (143).
 28. The gas sensor as claimed inclaim 1, wherein said ion-leakage preventing ceramic spacer (143) ismade mainly of alumina.
 29. The gas sensor as claimed in claim 1,wherein a thickness of the second oxygen-ion conductive solidelectrolyte layer (121) is 30–400 micrometers.
 30. The gas sensor asclaimed in claim 1, wherein a phase formed in the zirconia contained inthe first and/or second oxygen-ion conductive solid electrolyte layersconsists essentially of a cubic phase and tetragonal phase, with a phaseratio of cubic phase to tetragonal phase being from 1:4 to 2:1.
 31. Thegas sensor as claimed in claim 1, a content of the alumina contained inthe alumina substrate (11) is at least 70% by weight.
 32. A gas sensor(1 a) having a laminate comprising: an alumina substrate (11) having aheating resistor (115) embedded in the alumina substrate (11); a firstoxygen-ion conductive solid electrolyte layer (131) containing zirconiaand alumina and partly constituting an oxygen-detecting cell (13) andsaid first solid electrolyte layer (131) being laminated with saidalumina substrate (11); a second oxygen-ion conductive solid electrolytelayer (121) containing zirconia and alumina and partly constituting anoxygen-pumping cell (12); an ion-leakage preventing ceramic spacer (143)for preventing oxygen-ions from leaking from the second oxygen-ionconductive solid electrolyte layer (121) to the first oxygen-ionconductive solid electrolyte layer (131), said spacer (143) beinglaminated between said first and second oxygen-ion conductive solidelectrolyte layers (131, 121); and a gas-diffusion space (141) formedbetween an electrode (133) of the oxygen-detecting cell (13) and anelectrode (126) of the oxygen-pumping cell (12), wherein the first andsecond oxygen-ion conductive solid electrolyte layers contain aluminagrains; wherein the laminate is a co-fired laminate; wherein the firstand second oxygen-ion conductive solid electrolyte layers (131, 121)contain 10–80% by weight of alumina, respectively, and an average grainsize of alumina contained in the first and/or second oxygen-ionconductive solid electrolyte layers (131, 121) is less than 1micrometer; and wherein the second oxygen-ion conductive solidelectrolyte layer (121) contains alumina in an amount less than that ofthe first oxygen-ion conductive electrolyte layer (131).
 33. The gassensor as claimed in claim 32, wherein the second oxygen-ion conductivesolid electrolyte layer (121) contains alumina in an amount that is atleast 5% by weight less than that of the first oxygen-ion conductiveelectrolyte layer (131).
 34. The gas sensor as claimed in claim 32,wherein the second oxygen-ion conductive solid electrolyte layer 121constituting the oxygen-pumping cell (12) contains 60 to 90% by weightof zirconia and 10–40% by weight of alumina, and wherein the secondoxygen-ion conductive solid electrolyte layer (121) constituting theoxygen-pumping cell (12) contains alumina in an amount that is 10 to 50%by weight less than that of the first oxygen-ion conductive electrolytelayer (131) constituting the oxygen-detecting cell (13).
 35. The gassensor as claimed in claim 32, further comprising an electrode (136) anda lead (137), wherein the electrode (136) constituting theoxygen-concentration detecting cell (13) and facing the aluminasubstrate (11) is a reference electrode capable of storing oxygentherein and communicating with an atmosphere outside the sensor (1 a)through a lead (137) connected to the electrodes (136), said lead (137)serving as a channel (16) for draining oxygen.
 36. The gas sensor asclaimed in claim 32, further comprising a reduction-preventinginsulative layer (128) for preventing deoxidation of the secondoxygen-ion conductive layer (121) around the lead (127) of theoxygen-pumping cell (12), said reduction-preventing insulative layer(128) being provided between a lead (127) of the oxygen-pumping cell(12) and the oxygen-ion conductive solid electrolyte layer (121).
 37. Agas sensor (1 a) having a laminate comprising: an alumina substrate (11)having a heating resistor (115) embedded in the alumina substrate (11);a first oxygen-ion conductive solid electrolyte layer (131) containingzirconia and alumina and partly constituting an oxygen-detecting cell(13) and said first solid electrolyte layer (131) being laminated withsaid alumina substrate (11); a second oxygen-ion conductive solidelectrolyte layer (121) containing zirconia and alumina and partlyconstituting an oxygen-pumping cell (12); an ion-leakage preventingceramic spacer (143) for preventing oxygen-ions from leaking from thesecond oxygen-ion conductive solid electrolyte layer (121) to the firstoxygen-ion conductive solid electrolyte layer (131), said spacer (143)being laminated between said first and second oxygen-ion conductivesolid electrolyte layers (131, 121); and a gas-diffusion space (141)formed between an electrode (133) of the oxygen-detecting cell (13) andan electrode (126) of the oxygen-pumping cell (12), wherein the firstand second oxygen-ion conductive solid electrolyte layers containalumina grains, wherein the laminate is a co-fired laminate; wherein thesecond oxygen-ion conductive solid electrolyte layer (121) constitutingthe oxygen-pumping cell (12) contains 60 to 90% by weight of zirconiaand 10–40% by weight of alumina; wherein the first oxygen-ion conductivesolid electrolyte layer (131) constituting the oxygen-detecting cell(13) contains 40 to 80% by weight of zirconia and 20–60% by weight ofalumina; and wherein the second oxygen-ion conductive solid electrolytelayer (121) constituting the oxygen-pumping cell (12) contains aluminain an amount that is 10 to 50% by weight less than that of the firstoxygen-ion conductive electrolyte layer (131) constituting theoxygen-detecting cell (13).
 38. The gas sensor as claimed in claim 37,further comprising an electrode (136) and a lead (137) wherein theelectrode (136) constituting the oxygen-concentration detecting cell(13) and facing the alumina substrate (11) is a reference electrodecapable of storing oxygen therein and communicating with an atmosphereoutside the sensor (1 a) through a lead (137) connected to theelectrodes (136), said lead (137) serving as a channel (16) for drainingoxygen.
 39. The gas sensor as claimed in claim 37, further comprising areduction-preventing insulative layer (128) for preventing deoxidationof the second oxygen-ion conductive layer (121) around the lead (127) ofthe oxygen-pumping cell (12), said reduction-preventing insulative layer(128) being provided between a lead (127) of the oxygen-pumping cell(12) and the oxygen-ion conductive solid electrolyte layer (121).
 40. Agas sensor (1 a) having a laminate comprising: an alumina substrate (11)having a heating resistor (115) embedded in the alumina substrate (11);a first oxygen-ion conductive solid electrolyte layer (131) containingzirconia and alumina and partly constituting an oxygen-detecting cell(13) and said first solid electrolyte layer (131) being laminated withsaid alumina substrate (11); a second oxygen-ion conductive solidelectrolyte layer (121) containing zirconia and alumina and partlyconstituting an oxygen-pumping cell (12); an ion-leakage preventingceramic spacer (143) for preventing oxygen-ions from leaking from thesecond oxygen-ion conductive solid electrolyte layer (121) to the firstoxygen-ion conductive solid electrolyte layer (131), said spacer (143)being laminated between said first and second oxygen-ion conductivesolid electrolyte layers (131, 121); and a gas-diffusion space (141)formed between an electrode (133) of the oxygen-detecting cell (13) andan electrode (126) of the oxygen-pumping cell (12), wherein the firstand second oxygen-ion conductive solid electrolyte layers containalumina grains; wherein the laminate is a co-fired laminate; furthercomprising an ionic migration-preventing electrode (117) for preventingthe heating resistor (115) from deterioration or electricaldisconnection, wherein an electric potential of said ionicmigration-preventing electrode (117) is equal to or lower than thelowest electric potential of any part of the heating resistor (115); andwherein the second oxygen-ion conductive solid electrolyte layer (121)contains alumina in an amount less than that of the first oxygen-ionconductive electrolyte layer (131).
 41. The gas sensor as claimed inclaim 40, wherein the second oxygen-ion conductive solid electrolytelayer (121) constituting the oxygen-pumping cell (12) contains 10–80% byweight of alumina.
 42. The gas sensor as claimed in claim 40, whereinthe first oxygen-ion conductive solid electrolyte layer (131)constituting the oxygen-concentration cell (13) contains 10–80% byweight of alumina.
 43. The gas sensor as claimed in claim 40, furthercomprising a reduction-preventing insulative layer (128) for preventingdeoxidation of the second oxygen-ion conductive layer (121) around thelead (127) of the oxygen-pumping cell (12), said reduction-preventinginsulative layer (128) being provided between a lead (127) of theoxygen-pumping cell (12) and the oxygen-ion conductive solid electrolytelayer (121).
 44. The gas sensor as claimed in claim 40, wherein thesecond oxygen-ion conductive solid electrolyte layer (121) containsalumina in an amount that is at least 5% by weight less than that of thefirst oxygen-ion conductive electrolyte layer (131).
 45. The gas sensoras claimed in claim 40, wherein said ionic migration-preventingelectrode (117) is formed between an outer surface of the aluminasubstrate (11) and the heating resistor (115).
 46. The gas sensor asclaimed in claim 45, wherein the second oxygen-ion conductive solidelectrolyte layer (121) constituting the oxygen-pumping cell (12)contains 10–80% by weight of alumina.
 47. The gas sensor as claimed inclaim 45, wherein the first oxygen-ion conductive solid electrolytelayer (131) constituting the oxygen-concentration cell (13) contains10–80% by weight of alumina.
 48. The gas sensor as claimed in claim 45,further comprising a reduction-preventing insulative layer (128) forpreventing deoxidation of the second oxygen-ion conductive layer (121)around the lead (127) of the oxygen-pumping cell (12), saidreduction-preventing insulative layer (128) being provided between alead (127) of the oxygen-pumping cell (12) and the oxygen-ion conductivesolid electrolyte layer (121).
 49. The gas sensor as claimed in claim45, wherein the second oxygen-ion conductive solid electrolyte layer(121) contains alumina in an amount that is at least 5% by weight lessthan that of the first oxygen-ion conductive electrolyte layer (131).